High-resolution digital input imaging processes are desirable for superior quality printing applications, especially those requiring that changes be made from one print to the next or those where relatively short numbers of prints are to be made. As is well known, such processes may include electrostatographic processes using small-particle dry toners, e.g., having particle diameters less than about 7 micrometers, electrostatographic processes using solvent based liquid developers (also referred to as liquid toners) in which the particle size is typically on the order of 0.1 micrometer or less, and ink-jet processes using aqueous or solvent based inks.
The most widely used high-resolution digital commercial electrostatographic processes involve electrophotography. Although capable of high process speeds and excellent print quality, electrophotographic processes using dry or liquid toners are inherently complicated, and require expensive, large, complex equipment. Moreover, due to their complex nature, electrophotographic processes and machines tend to require significant maintenance.
Ink jet technology may be used to deposit fluid materials on substrates and has numerous applications, mainly in printing. However, to avoid running and smearing of the ink droplets, the paper used in an ink jet printer must be porous, thereby restricting the papers that can be used and virtually eliminating the use of high quality graphic arts papers. In addition, the absorption of the ink by the paper limits the density of the images that can be produced. Finally, drying of ink requires a large amount of energy and would produce an inordinate amount of water or solvent vapors if used in high volume print engines. In addition, to avoid clogging ink jet heads, most ink jet inks include a dye dispersed in a solvent such as water or alcohol. However, dyes are subject to fading. Pigments are more resistant to fading, but are particulate and tend to clog ink jet heads. To avoid clogging, larger nozzles can be made. This however, results in larger ink droplets being formed, thereby reducing image resolution and quality.
Ink jet printing suffers from a number of drawbacks. Ink jet printing is typically slower than traditional offset printing. This is especially true for process color printing. For example, the linear printing speed of ink jet printing is typically of the order of 10 times slower than can be achieved in offset printing. This represents a major issue limiting the implementation of ink jet technology in industrial printing systems. The ink jet printing speed limit is dictated by the rate at which ink jet nozzles can eject ink in discrete controllable amounts. This rate is at present on the order of 20,000 pulses per second for drop-on-demand (DOD) ink jet printers. This limits state of the art DOD ink jet printers to print rates on the order of 2 pages per second. Continuous ink jet printing can be performed more quickly. However, at high speeds, the results tend to be poor due to the difficulties mentioned above.
Another limitation of printing at high speed with ink jet technology arises from the amount of liquid used in ink jet printing. Ink jet inks typically have a low concentration of colorant, predicated by the fine density variations required for producing good image quality. Thus, the image on the receiver has relatively large amounts of ink, which need to be dried before the image is usable. At high speeds, this drying step is complex and energy-intensive.
Ink jet printing currently cannot achieve printing quality as high as can be achieved using offset printing techniques. Ink jet printing is often characterized by a distinctive banding pattern that is repeated over the printed image. This may be traced to the arrangement of the ink jet nozzles in the printing head. Relatively small nozzle misalignments or off-center emission of droplets can cause banding. As the printing head is translated laterally across the width of the printing surface, the visual imperfections are periodically repeated. This produces banding or striping which is characteristic of ink jet printers. A number of approaches exist to control banding. These approaches reduce throughput of the printer.
Print quality of ink jet printers is also reduced by “wicking” or “running” of the ink jet inks. The low-viscosity inks typically employed in ink jet printers tend to “run” along the fibers of certain grades of paper. This phenomenon leads to reduced quality printing, particularly on the grades of paper desirable in high-volume printing. Wicking can cause printed dots to become much larger than the droplet of ink emerging from the ink jet nozzle. Wicking can also reduce the brightness of the image, as the some of the colorant in the image gets wicked below the surface, thus not contributing adequately to image brightness.
It is possible to reduce wicking by printing on specially treated paper receivers. However, such paper tends to be undesirably expensive. Furthermore, in order to produce prints that resemble photographic prints, a type of receiver that is commonly used has a polymer layer to mimic the resin-coated photographic paper. As polymers do not absorb water or the carrier fluid of ink, the polymer layer has to incorporate voids or channels to “absorb” the relatively large amount of ink in a typically high-coverage pictorial image, which increases the cost and complexity of the receiver.
The matter of failure in ink jet nozzles is also deserving of attention. Various approaches exist for detecting faulty ink jet nozzles and for readdressing the ink jet printing head to permit other nozzles to perform the tasks of faulty nozzles. This includes various redundancy schemes. Again, these usually have the effect of slowing down the net printing process speed. In many cases the redundancy is managed at printing head, requiring backups for entire printing heads. This adds to the cost of the technology per printed page and again limits the industrial implementation of the technology.
Another important problem is the presence of fluid in the image. Prior art describes forming the image on an intermediate, then transferring the image to a receiver. U.S. Pat. No. 5,099,256 discloses the use of a cylinder specifically coated with a silicone polymeric material in combination with a drop-on-demand print head. U.S. Pat. No. 6,736,500 discloses the use of a coagulating agent that increases the viscosity of the ink jet ink to improve transfer and image durability. U.S. Pat. Nos. 6,755,519 and 6,409,331 teach methods for increasing ink viscosity such as via UV cross-linking or evaporation. None of these patents address the formation of a multi-color image.
U.S. Pat. Nos. 6,761,446; 6,767,092; 6,719,423; and 6,761,446 refer to forming images on separate intermediates, then transferring the images in register to form a four-color image on a receiver. While these patents address the problem of excess fluid in a four-color image, the process of registration of the component images from separate intermediates involve complex and expensive mechanisms. The situation is further complicated if receivers of different thickness and/or surface properties need to be used. In addition, the receiver path to accommodate successive transfers to form the multi-color image is relatively long, affecting cost and reliability.
Thus, there remains a need for a simpler method of using ink jet printing to form high quality color images on a wide range of substrates, without the aforementioned limitations of prior art. In addition, there is a need for ink jet printing methods that provide combinations of print quality, speed, and cost which improve on the prior art.
Gravure printing is a well-known commercial process in which gravure ink is applied to a plate or roller, including a multitude of individual cells, corresponding to the image that is desired to be printed. Ink is applied via an applicator that typically has a doctor blade. A receiver (typically paper) is then pressed against the inked image and some of the ink, typically about 60% in each cell, is transferred to the receiver. An electrostatic field may be applied across the transfer nip to enhance transfer.
In order for a gravure ink to uniformly coat a gravure roller or plate (hereafter referred to as a gravure roller or gravure cylinder, with the understanding that either term is inclusive of a gravure plate), the viscosity of a gravure ink ranges from roughly 50 to 1,000 cpoise (measured under low shear conditions).
Gravure printing is ideal for high run length printing applications, but is not generally suitable for shorter runs. There are several reasons for this. Firstly, a gravure cylinder is made to correspond specifically to the image that is being printed. This is time consuming and expensive and must be amortized over many prints to yield suitable low cost prints. Secondly, there is no way to ink the roller in a fashion that would enable it to print variable data, such as would be the case in digital printing. Finally, gravure printing leaves approximately 40% of the ink behind in the gravure cylinder. This would create printing artifacts such as ghost images if the roller were used for variable data printing, unless the roller was first thoroughly cleaned. Cleaning the gravure roller thoroughly is a difficult but necessary process since any trace amounts of ink remaining within a cell, normally inconsequential in conventional gravure printing because the same image is printed repeatedly, is quite detrimental to subsequent prints where variable data streams are involved.
U.S. Pat. Nos. 6,767,092; 6,761,446; 6,719,423; and 6,682,189 disclose a device that prints an ink jet image onto the surface of an imaging member, fractionates the ink particles from the liquid, then removes some or all of the liquid before transferring the ink to paper. Devices of this type may lead to image blurring from liquid coagulation, or dot placement errors and satellites from the ink jet device. There is also a need to formulate separate pigmented inks for each color, leading to concerns about interactions between pigment particles and the ink jet print head since the ink jet device uses the different pigmented liquid for each color.